Recently, high efficiency transmission through digitalization has become widely used in wireless communication. For this type of high efficiency transmission, techniques to linearize power amplifier amplification characteristics to suppress non-linear distortion and reduce adjacent channel leakage power at transmission side devices such as base stations are important.
A pre-distortion method is known as one such distortion compensation method (see, for example, Japanese Laid-open Patent Publication No. 2005-65211 and Japanese Laid-open Patent Publication No. 2007-221245). In the pre-distortion method, a desired signal without distortion of the amplifier output is obtained by adding characteristics that are opposite the amplifier distortion characteristics to the amplifier input signal.
Specifically, the distortion compensation process based on the pre-distortion method involves comparing a transmission signal before the distortion compensation with a demodulated feedback signal, using the difference (power difference) from the comparison to calculate a distortion compensation coefficient, and then using the calculated distortion compensation coefficient to update the prior distortion compensation coefficient. The distortion compensation coefficient is stored in a memory by using transmission signal amplitude, power, or functions thereof as an address. Then, the transmission signal gain is adjusted to meet inverse characteristics of the distortion characteristics of the power amplifier by multiplying the updated distortion compensation coefficient by the next transmission signal to be sent. The gain adjusted transmission signal is then inputted into the power amplifier. By repeating this operation, finally the signals converge into an optimal distortion compensation coefficient and the distortion of the power amplifier is compensated.
Transmission frequencies (carrier frequencies) of wireless communication devices have specific bands for accommodating the various users who use wireless communication devices. Accordingly, a user can specify a desired transmission frequency in a wireless communication device according to the area and the like in which the user is using the wireless communication device.